The present invention relates to a thermosetting coating composition, and more particularly to a thermosetting coating composition showing good dispersibility of pigments, usable as a high-solid coating composition, capable of providing a coated layer having excellent acid resistance and scuff (mar) resistance as well as good weathering resistance, substantially free from the harmfulness of a polyisocyanate compound, which is suitable as a topcoat composition for automobiles, and as a coating composition for coils, home electric appliances, electric parts, etc.
Topcoats of automobiles are generally made of a combination of a hydroxyl group-containing polymer and a melamine resin as a curing agent. Coated layers produced from the coating composition containing a melamine resin as a curing agent generally have good surface hardness, but show poor acid resistance. Accordingly, such coated layers are susceptible to coating defects caused by acid rain, etc. The poor acid resistance is presumably due to the presence of a triazine ring in the melamine resin.
In view of this fact, development has been conducted to provide a coating composition containing no melamine resin. For instance, a curing agent using a polyisocyanate compound has been proposed. However, handling of a coating composition containing a polyisocyanate compound as a curing agent requires much attention with respect to safety.
Also, a proposal has been made to provide a high-solid, thermosetting coating composition containing a low-molecular weight polyoxide, a low-molecular weight hydroxyl group-containing polyfunctional material, a curing agent consisting essentially of an acid anhydride, and an acid catalyst (U.S. Pat. No. 4,755,581). However, since this coating composition contains a curing agent showing too high a reactivity, it is difficult to store this coating composition in a one-pack system, and its coating operation is hard to conduct.
As a coating composition which has overcome the difficulty of handling of polyisocyanate and is usable in a one-pack system, EP 353,734 discloses a thermosetting coating composition containing a polymer in which acid anhydride groups are subjected to ring opening and half-esterification (one of two branches produced by the ring opening is esterified), a hydroxy group-containing compound, and an epoxy compound. A coated layer obtained from this coating composition shows improved acid resistance, but it still shows insufficient water resistance and weathering resistance, etc. It is also a little poor in scuff (mar) resistance.
With respect to the topcoats of automobiles, they are required to have good dispersibility of pigments in addition to the above acid resistance, scuff (mar) resistance, water resistance and weathering resistance. Also, to prevent air pollution, it is required that a coating composition has as high a solid content as possible to reduce its solvent content.